1 Copyright © 2016 McGraw-Hill Education. All rights reserved. No reproduction or distribution...

1Copyright © 2016 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education .

Chapter 03

Lecture Outline

3.1 Mendel’s Study of Pea Plants

Why pea plants are suitable for genetic studies

The steps that Mendel followed to make crosses between different strains of pea plants

The seven characteristics of pea plants that Mendel chose to study

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• Before Mendel, people knew that parents passed traits onto offspring – but they didn’t understand how it worked

• Some early theories of inheritance:– Pangenesis

• Hippocrates• “Seeds” produced by all parts of body, collected

and transmitted to offspring at conception– Blending hypothesis

• Factors that control hereditary traits are malleable• They blend together generation after generation


3

Early Theories of Inheritance

• These theories were refuted by the work of Gregor Mendel in the mid-1800’s

• Mendel’s work was novel– He used quantitative analysis – He developed general laws – rules to predict

• which phenotypes would appear in offspring• ratios of phenotypes in the offspring

• His work was first ignored, then rediscovered in the early 1900’s


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• The pea (Pisum sativum) has several advantages:

– Small, easily grown

– Each flower has male and female structures• A plant can fertilize itself (selfing)• OR, A plant can be crossed to another different plant

(cross fertilization)

– Many different varieties were available with different traits


5

Mendel’s Choice of the Pea Plant

Figure 3.2 a

(a) Structure of a pea flower

Petals

Keel

Stigma

Anther

Style

Ovary

Sepal

Ovule

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Pollen lands on the stigma

Anthers contain pollen grains, where the male gametes are produced

• Variable characters of pea plants:– Height– Flower color– Flower position– Seed color and shape– Pod color and shape


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Mendel Studied 7 CharactersCHARACTER VARIANTS

Flower color

Purple White

Flower position

Axial Terminal

Height

Tall Dwarf

CHARACTER VARIANTS

Seed colorYellow Green

Seed shapeRound Wrinkled

Pod shape

Smooth Constricted

Pod color

YellowGreen

Remove anthersfrom purple flower.

Anthers

Transfer pollenfrom anthers ofwhite flower tothe stigma of apurple flower.

Cross-pollinated flowerproduces seeds.

Plant the seeds.

White

PurpleParentalgeneration

First-generationoffspring

• Mendel carried out two types of crosses

– Self-fertilization• Pollen and egg are derived from

the same plant

– Cross-fertilization• Pollen and egg are derived from

different plants• When plants with different traits

are crossed, this is hybridization – progeny are called hybrids

• To cross-fertilize, Mendel transferred pollen into the flower of another plant

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• Mendel started with plants that “bred true” for different character

• True-breeding lines – Plants that always produce progeny with the same traits when self-fertilized (or bred to the same strain)

• A note on terminology:– Character – The type of characteristic that can vary,

such as “height”– Trait, or variant – The version of the character, such

as “tall” or “dwarf”


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True-Breeding Lines

3.2 Law of Segregation

Mendel’s experiments with single-factor crosses

The law of segregation and how it is related to gamete formation and fertilization

Predicting outcomes of single-factor crosses using a Punnett square


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• Mendel did not start with a hypothesis to explain the formation of hybrids– But he believed that a quantitative analysis of crosses

may reveal a mathematical relationship– This is called an empirical approach– General findings from such an approach are called

empirical laws


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Mendel’s Approach

• Mendel mated true-breeding plants with one trait to plants with a different trait to create hybrids

– Matings looking at one character – single-factor cross

– Matings looking at two characters – two-factor cross


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Mendel’s Crosses

Mendel’s Single-Factor Cross Experiments

• Mendel studied seven characters, each with two variants– e.g., Plant height variants were tall and dwarf

• His first experiments crossed only two variants of one character at a time– Called a single-factor cross or monohybrid cross

• He followed the characters for two subsequent crosses– P generation – Parental generation– F1 generation – 1st Filial generation

– F2 generation – 2nd Filial generation


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Experimental level

P plants

Tall Dwarf

F1 seeds

Self-fertilization

Alltall

Self-fertilization

F1 plants

F2 seeds

F2 plants

Conceptual level

TT + 2 Tt + tt

All Tt

Tt

x

x

TT tt

Note: The Pcross producesseeds that arepart of the F1

generation.

Tall Tall TallDwarf

1. For each of seven characters, Mendelcross-fertilized two differenttrue-breeding strains. Keep in mindthat each cross involved two plantsthat differed in regard to only one ofthe seven characters studied. Theillustration at the right shows onecross between a tall and dwarf plant.This is called a P (parental) cross.

2. Collect the F1 generation seeds. The following spring, plant the seeds and allow the plants to grow. These are the plants of the F1 generation.

3. Allow the F1 generation plants toself-fertilize. This produces seeds

thatare part of the F2 generation.

4. Collect the F2 generation seeds and plant them the following spring to obtain the F2 generation plants.

5. Analyze the traits foundin each generation.

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P Cross F1 generation F2 generation Ratio

Tall X dwarf stem

All tall 787 tall 277 dwarf

2.84:1

Purple X white flowers

All purple 705 purple224 white

3.15:1

Axial Xterminal flowers

All axial 651 axial207 terminal

3.14:1

Yellow X Green seeds

All yellow 6,022 yellow2,001 green

3.01:1

Round Xwrinkled seeds

All round 5,474 round1,850 wrinkled

2.96:1

Green X yellow pods

All green 428 green152 yellow

2.82:1

Smooth X constricted pods

All smooth 882 smooth229 constricted

2.95:1

TOTAL All dominant 14,949 dominant5010 recessive

2.98:1

DATA FROM MONOHYBRID CROSSES


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• For all seven characters studied – The F1 generation showed only one of the two parental

traits – The F2 generation showed an ~ 3:1 ratio of the two

parental traits

• These results refuted a blending mechanism of heredity– The recessive trait “disappeared” entirely in the F1

– But reappeared unchanged in the F2

• The data suggested a particulate theory of inheritance


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Interpreting the Data

• Dominant and recessive traits:– The trait that is exhibited in the F1 is called dominant

– The trait that is masked in the F1 is called recessive

• In the F1, only the dominant trait appeared

• In the F2, the dominant trait plants outnumbered recessive trait plants with a 3:1 ratio


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1. For a given character, a pea plant contains two discrete hereditary factors, one from each parent

2. The two factors may be identical or different

3. When the two factors of a single character are different– One is dominant and its effect can be seen– The other is recessive and is not expressed

4. During gamete formation, the paired factors segregate randomly so that half of the gametes receive one factor and half of the gametes receive the other– This is Mendel’s Law of Segregation


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Mendel postulated:

The two copies of a gene segregate (or separate)

from each other during transmission

from parent to offspring


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Mendel’s Law of Segregation

– Genes – the modern term for Mendelian factors

– Alleles – different versions of the same gene

– Homozygous – an individual with two identical alleles

– Heterozygous – an individual with two different alleles

– Genotype – an individual’s specific allelic composition

– Phenotype – the outward appearance of an individual


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Terminology

Figure 3.6

P generation

Segregation

Segregation

Self- fertilization

Cross-fertilization

F2 generationGenotypes:(1 : 2 : 1)

Phenotypes:(3 : 1)

Gametes

F1 generation(all tall)

Gametes

tT t

t TT t

x

Tall

Tall

TT

Tt

Dwarf

tt

Tt TtTT tt

Tall Tall Tall Dwarf

T


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Punnett Squares Are Used to Predict the Outcome of Crosses

• A Punnett square is a grid that enables one to predict the outcome of simple genetic crosses– Proposed by the English geneticist, Reginald Punnett

• Must know the genotype of the parents

• We will illustrate the Punnett square approach using the cross of heterozygous tall plants as an example


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1. Write down the genotypes of both parents

Male parent = Tt

Female parent = Tt

2. Write down the possible gametes each parent can make

Male gametes: T or t

Female gametes: T or t

3. Create an empty Punnett square

4. Fill in the possible genotypes of the offspring

Using a Punnett Square


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Punnett square of a cross between two heterozygotes for one character

TT Tt

Tt tt

Male gametes

Fem

ale

gam

etes

T

T

t

t


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5. Determine proportions of genotypes and phenotypes– Genotypic ratio

• TT : Tt : tt• 1 : 2 : 1

– Phenotypic ratio• Tall : dwarf• 3 : 1


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TT Tt

Tt tt

Male gametes

Fe

ma

le g

am

ete

s

T

T

t

t

3.3 Law of Independent Assortment

Mendel’s experiments involving two-factor crosses

The law of independent assortment

Predicting the outcome of two-factor crosses using a Punnett square


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Mendel’s Two-Factor Cross Experiments

• Mendel also performed two-factor crosses– Crossing individual plants that differ in two characters

• Example:– Character 1 = Seed shape (round vs. wrinkled)– Character 2 = Seed color (yellow vs. green)

• There are two possible patterns of inheritance for these characters – either linked or independent assortment

• Refer to Figure 3.7


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Figure 3.7


P generation

Haploid gametes

RRYY

RY x

rryy

RrYyF1 generation

Haploid gametes RY ry

ry

(a) HYPOTHESIS: Linked assortment

1/21/2 Haploid gametes

RRYY

RY x

rryy

RrYy

ry

Ry ryrYRY

(b) HYPOTHESIS: Independent assortment

1/41/4

1/41/4

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Figure 3.8

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P Cross F1 generation F2 generation

Round,yellow seeds X wrinkled, green seeds

All round, yellow 315 round, yellow seeds101 wrinkled, yellow seeds108 round, green seeds 32 green, wrinkled seeds

DATA FROM DIHYBRID CROSSES


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Interpreting the Data

• The F2 generation contains seeds with novel combinations not found in the parental generation– Round and green– Wrinkled and yellow

• These nonparentals are predicted if the genes are segregating independently of each other


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P Cross F1 generation F2 generation Ratio

Round,yellow seeds X wrinkled, green seeds

All round, yellow 315 round, yellow seeds101 wrinkled, yellow seeds108 round, green seeds 32 green, wrinkled seeds

9.83.23.41.0

Predicted phenotypic ratio in the F2 generation would be 9:3:3:1 if genes act independently of each other


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During gamete formation, the segregation of any pair of hereditary determinants is independent of

the segregation of other pairs


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Mendel’s Law of Independent Assortment

Methods for Independent Assortment Problems

• Like a one-factor cross, a two-factor cross can be displayed as an array diagram – Refer to Figure 3.9

• Punnett squares can also be used to predict the outcome of crosses involving two independently assorting genes– Refer to Figure 3.10


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Figure 3.9


Four possible malegametes:

Four possible femalegametes:

RY Ry ry

ry

rY

rYRyRY

RRYY RRYy RrYY RrYy RRYy RRyy RrYy Rryy RrYY RrYy rrYY RrYy Rryy rrYy rryy

By randomly combining male and female gametes, 16 combinations are possible.

Totals: 1 RRYY : 2 RRYy : 4 RrYy : 2 RrYY : 1 RRyy : 2 Rryy : 1 rrYY : 2 rrYy : 1 rryy

Phenotypes:

rrYy

9 round,yellow seeds

3 round,green seeds

3 wrinkled,yellow seeds

1 wrinkled,green seed

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Figure 3.10


Cross: TtYy x TtYy

TY

TY

Ty

tY

ty

Genotypes: 1 TTYY : 2 TTYy : 4 TtYy : 2 TtYY :

TTYY TTYy TtYY TtYy

Tall, yellow Tall, yellow Tall, yellow Tall, yellow

TtyyTtYyTTyyTTYy

TtYY TtYy ttYY ttYy

ttyyttYyTtyyTtYy

Tall, yellow Tall, yellow

Tall, yellow

Tall, yellow Tall, yellow Tall, greenTall, green

Tall, green

Dwarf, yellowDwarf, yellow

Dwarf, yellow Dwarf, green

Phenotypes:

1 TTyy : 2 Ttyy

9 tallplants with

yellow seeds

3 tallplants with

green seeds

3 dwarfplants with

yellow seeds

1 dwarfplant with

green seeds

1 ttYY : 2 ttYy 1 ttyy

Ty tY ty

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• In crosses involving three or more independently assorting genes, a single Punnett square becomes cumbersome– Would need 64 squares for three genes!– Can use three Punnett Squares plus

the multiplication method– Refer to Figure 3.11a, b

• A second alternative is the forked-line method– Refer to Figure 3.11c


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Three-factor crosses


38Figure 3.11a, b


39Figure 3.11c

3.4 Chromosome Theory of Inheritance

The key tenets of the chromosome theory of inheritance

The relationship between meiosis and Mendel’s laws of inheritance


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Chromosome Theory of Inheritance

• A major breakthrough in our understanding of genetics

• Established the framework for understanding how chromosomes carry and transmit genetic determinants

• Explains the patterns of inheritance seen by Mendel

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• Chromosome Theory of Inheritance resulted from three lines of evidence:

1. Mendel’s breeding experiments

2. Nägeli and Weismann• A substance in living cells is responsible for inherited traits• Parents contribute equally to determine traits of offspring• Hertwig, Strasburger, and Flemming suggested that

chromosomes are the carriers of the genetic material

3. Boveri and Sutton • Saw similarity between segregation of traits and behavior

of chromosomes during meiosis• Proposed the chromosome theory of inheritance

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Inheritance patterns of traits can be explained by transmission patterns of chromosomes

during meiosis and fertilization


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Chromosome Theoryof Inheritance


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Chromosome Theory of Inheritance

1. Chromosomes contain the genetic material

2. Chromosomes are replicated and passed from parent to offspring

• Also from cell to cell during development

• Chromosomes retain individuality during transmission

3. Nuclei of most eukaryotic cells contain chromosomes in homologous pairs (they are diploid)

• Gametes, however, are haploid

4. In the formation of haploid cells, chromosomes segregate independently

5. Each parent contributes one set of chromosomes

Law of Segregation is Explained by Separation of Homologs

• Mendel’s Law of Segregation can be explained by the separation of homologous chromosomes during meiosis

• Consider a situation where one homolog carries a dominant allele (Y, yellow seeds) and the other carries the recessive allele (y, green seeds)– The gametes of the heterozygote may contain the

dominant allele or the recessive allele, but not both

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Law of Independent Assortment is Explained by Random Alignment of Homologs

• Mendel’s Law of Independent Assortment can be explained by the random alignment of homologous chromosomes during meiosis

• Consider a situation where a double heterozygote carries the dominant and recessive alleles for two genes, each gene on a different chromosome– The chromosomes with dominant alleles may end up together

in a gamete, or not– All four combinations are possible in the gametes

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3.5 Studying Inheritance Patternsin Humans

The features of a pedigree

Analysis of a pedigree to determine if a trait or disease is dominant or recessive


49

Pedigree Analysis

• When studying human traits, it is not ethical to control parental crosses (as Mendel did with peas)– So we must infer gene

properties from analysis of family trees or pedigrees

Female

Male

Sex unknown or notspecified

Miscarriage

Deceased individual

Unaffected individual

Affected individual

Presumed heterozygote(the dot notation indicatessex-linked traits)

Consanguineous mating(between related individuals)

Fraternal (dizygotic) twins

Identical (monozygotic) twins

I -1 I-2

III -4 II -5II -1 II -2 II -3

III -3III -1 III -2 III -6 III -7III -4 III -5

(a) Human pedigree showing cystic fibrosis

(b) Symbols used in a human pedigree

Pedigree Analysis

• Pedigree analysis is commonly used to determine the inheritance pattern of human genetic diseases

• Genes that play a role in disease may exist as– A normal allele – A mutant allele that causes disease symptoms

• Diseases can follow a simple Mendelian pattern of inheritance that is either dominant or recessive


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• Recessive pattern of inheritance– Two unaffected heterozygous individuals will

on average have 25% affected offspring– Two affected individuals will have 100% affected offspring– Can “skip generations”

• Dominant pattern of inheritance– Does not skip generations– Affected individual will have at least one affected parent

• However, disease may also result from a new mutation


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Example: Cystic fibrosis (CF)

– A recessive disorder of humans

– Affected gene is the cystic fibrosis transmembrane conductance regulator (CFTR)

– The mutant CFTR protein causes ion imbalance• Leads to abnormalities in many tissues

and organs– pancreas, skin, intestine, sweat glands and lungs

• Buildup of sticky mucus in the lungs makes breathing difficult

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3.6 Probability and Statistics

Definition of probability

Predicting the outcome of crosses using the product rule and binomial expansion equation

Evaluating the validity of a hypothesis using a chi square test


54

Probability and Statistics

• The laws of inheritance can be used to predict the outcomes of genetic crosses

• For example:– Animal and plant breeders are concerned with the

types of offspring produced from their crosses– Parents are interested in predicting the traits that their

children may have• This is particularly important in the case of families

with genetic diseases


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Probability

• The probability of an outcome is the chance, or likelihood, that the outcome will occur

• Probability =


Total number of possible outcomes

• For example, in a coin flip

Number of times an outcome will occur

(1 heads + 1 tails) = 1/2 = 50%Pheads = 1 heads

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• In our pea genetics example:

• Probability =


Total number of individuals

Expected number of individuals with a given phenotype

(3 tall + 1 dwarf) = 3/4 = 75%Ptall = 3 tall

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(3 tall + 1 dwarf) = 1/4 = 25%Pdwarf = 1 dwarf

• The larger the size of the sample, or number of times the experiment is performed, the more closely the observed results will match the expected outcomes

• This is due to random sampling error– Random sampling error is large for small samples,

and small for large samples

• For example– If a coin is flipped only 10 times, it is not unusual to get

70% heads and 30% tails – If the coin is flipped 1,000 times the percentage of heads

will be fairly close to the predicted 50% value


58

Product rule

The probability that two or more independent events will

occur is equal to the product of their respective probabilities

• “Independent events” are those in which the occurrence of one does not affect the probability of another


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• Consider the disease congenital analgesia – Recessive trait in humans– Affected individuals can distinguish between sensations

• However, extreme sensations are not perceived as painful – they do not perceive pain

– Two alleles• P = Normal allele• p = Congenital analgesia

• Question:– Two heterozygous individuals plan to start a family– What is the probability that the couple’s first three children

will all have congenital analgesia?


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• Applying the product rule

– Step 1: Calculate the individual probabilities• This can be obtained via a Punnett square

1/4 (25%)P(congenital analgesia) =

– Step 2: Multiply the individual probabilities

1/4 X 1/4 X 1/4 = 1/64 = 0.016 = 1.6%

• This is the probability that the first three offspring will all exhibit the disease

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